The heat in an electric heating wire is transferred by radiation, conduction and convection. Especially from highly rated elements operating in air, if the environment is relatively cold, heat transfer by radiation is predominant. If radiation is the only means of transfer, Stefan Bolzman's law applies. Under certain assumptions it can be written as follows: EQU p=.epsilon..sup..multidot. .sigma..sup..multidot. (Te.sup.4 -Ts.sup.4)
where EQU .sigma.=5,670.times.10-8[W/m.sup.2 K] EQU p=surface rating [W/m.sup.2 ] EQU Te=element temperature EQU Ts=temperature of the environment EQU .epsilon.=emissivity coefficient of the surface of the heating element
(can have any value between 0 and 1)
This equation shows that for a certain surface rating (Te-Ts) reaches its lowest value when .epsilon. has its largest value, i.e.=1. In this case the surface is said to be radiating as a "perfectly black body". For ordinary materials .epsilon. varies from values which are as low as 0.005 for a bright metal surface, up to 0.9 for certain materials which also have a appropriate surface roughness. In order to achieve as low as possible element temperature at a predetermined surface rating, it will therefore be necessary to raise the emissivity coefficient of the material.